Metal-Organic Framework Fabricated V₂O₅ Cathode Material for High-Performance Lithium-Ion Batteries

Abstract

In this article, oval-shaped V₂O₅ nanoparticles were hydrothermally synthesized using a metal-organic framework (MOF), which was then followed by calcination under an air atmosphere. The obtained sample was characterized through various characterization techniques to determine the sample purity and the structural and morphological details. Since V₂O₅ possesses a layered crystal structure, it exhibits promising electrochemical performances as a cathode material for lithium-ion battery applications. However, poor cycling and inferior rate capabilities are the major issues that limit its application. Thus, a strategy to fabricate unique oval-shaped V₂O₅ nanoparticles was employed here to improve electrochemical performances using an MOF, which acts as a template and provides a skeleton for the growth of a novel nanostructure. It is believed that the oval-shaped morphology is beneficial to achieving better electrochemical results due to the large surface area and the existence of numerous channels for lithiation and de-lithiation. The obtained electrochemical result reveals that the V₂O₅ electrode can be considered a prominent cathode material for next-generation lithium-ion battery applications.

1. Introduction

As the modern development of society expanded, the demand for energy storage devices significantly increased. Currently, lithium-ion batteries (LIBs) are the most suitable energy storage devices, especially for portable electronics and electric vehicles [1,2,3,4,5]. Rechargeable LIBs are now globally used due to their environmentally friendly features, lightweight design, flexibility and high energy density. It is well-known that cathode materials largely affect not only the electrochemical performance but also the cost of LIBs. There are numerous commercially available cathode materials such as LiCoO₂ (140 mAh g⁻¹), LiMn₂O₄ (148 mAh g⁻¹) and LiFePO₄ (~170 mAh g⁻¹), which have the advantages of a high working voltage and an excellent cycling performance. However, their low theoretical capacities, as well as the high cost of materials, limit their practical applications [6,7,8,9,10,11,12]. Therefore, the development of a new class of cathode materials which exhibit a large specific capacity and high safety at a low cost is required.

To improve the performance of LIBs, transition metal oxides, phosphides, sulfides and various other materials have been widely used as cathode materials. Among these materials, transition metal oxides are considered potential cathode materials due to their physical and chemical properties, which help to obtain high specific capacities and an excellent cycling performance [13]. Vanadium pentoxide (V₂O₅), a potential transition metal oxide, has garnered great interest due to its mainly variable oxidation states (V²⁺, V³⁺, V⁴⁺ and V⁵⁺). V⁵⁺ is the most stable oxidation state of V₂O₅, and it is widely studied as a cathode material due to its low-cost, easy and scalable synthesis and high working voltage [14,15,16]. In addition, V₂O₅ has a layered crystal structure which allows it to accommodate the volume change during Li⁺ insertion/extraction efficiently, resulting in enhanced electrochemical performances [17]. The theoretical capacity of V₂O₅ is deeply affected by the amount of intercalated Li⁺ ions in the crystal structure, which eventually form Li_xV₂O₅ (0 < x < 2). However, the x value can be increased until 2 < x < 3 due to the continuous Li⁺ ion insertion at higher voltages [18,19]. As a cathode material for LIBs, the theoretical capacities of V₂O₅ of ~294 mAh g⁻¹ and ~440 mAh g⁻¹ can be obtained based on the intercalation of two and three Li⁺ ions, respectively, which are nearly three times higher than the theoretical capacity of LiCoO₂ (~140 mAh g⁻¹) [14]. Despite all these advantages, V₂O₅ still suffers from fast capacity fading due to the low lithium-ion diffusion efficiency, insufficient contact areas and poor structural stability [20,21,22]. For obtaining an enhanced electrochemical performance, higher electrode/electrolyte interfaces and a better accommodation ability of the strain upon volume change during lithiation/de-lithiation, nanostructured materials also possess advantages. Numerous nanostructured V₂O₅ samples have been reported, such as nanoparticles [23], micro/nanosphere [24,25,26], nanosheets [27,28,29], nanorods [30], nanotubes [31] and nanowires [32], to alleviate the kinetic limitations when employed as a cathode material for LIB.

Since metal-organic frameworks are the new class of porous inorganic-organic materials, demonstrating an alterable high specific surface area, framework topologies, active-metal sites, a crystalline structure and tunable pore sizes, they have attracted much attention for their promising applications in batteries and supercapacitors [33,34]. Therefore, in this work, we have synthesized oval-shaped V₂O₅ nanoparticles by using a simple and facile hydrothermal synthesis method. To synthesize a vanadium-based metal-organic framework, terephthalic acid as an organic moiety and vanadium acetylacetonate as a metal precursor were used. It is believed that the MOF provides a skeleton for the growth of an oval-shaped nanostructure. When V₂O₅ was applied as a cathode material for LIBs, it exhibited remarkable electrochemical performances.

2. Materials and Methods

2.1. Material Synthesis

Oval-shaped V₂O₅ nanoparticles were successfully synthesized using the facile hydrothermal synthesis method. Firstly, 2 mmol each of vanadium acetylacetonate (Sigma Aldrich, Saint Louis, MO, USA, 97%) and terephthalic acid (SRL, Mumbai, India, 98%) were dissolved in 60 mL of isopropanol with continuous stirring. The terephthalic acid is used here as an organic moiety to form a metal-organic framework. After continuous stirring for 45 min, the obtained homogeneous solution was then transferred to a Teflon-lined stainless-steel autoclave and heated at 180 °C for 12 h. After natural cooling, the obtained product was centrifuged and washed with distilled water/ethanol several times. After drying the sample at 75 °C overnight, the calcination was performed at 450 °C for 1 h in an air atmosphere.

2.2. Materials Characterisation

To discover the calcination temperature, the thermal analysis of the as-prepared sample was recorded using thermogravimetric/differential thermal analysis (TG/DTA) on a Shimadzu DTG 60 instrument (Shimadzu, Kyoto, Japan) from room temperature to 700 °C under an air atmosphere. The crystallinity, purity, particle sizes and morphology of the calcined sample were investigated using X-ray diffraction analysis (XRD, Bruker, D8 Discover High Resolution X-Ray Diffractometer with Cu Kα radiation, λ = 1.5406 Å, Karlsruhe, Germany) and field-emission transmission electron microscopy (FE-TEM, Philips TecnaiF20 at 200 kV, Philips, Amsterdam, The Netherlands). The elemental distribution analysis of the calcined sample was carried out by energy-dispersive X-ray spectrometry (EDX, Mode: JSM 6610LV, JEOL, Tokyo, Japan).

2.3. Electrode Fabrication

The testing electrodes were fabricated by mixing the active material of V₂O₅ (75 wt %), Ketjen black (15 wt %) and polyvinylidene fluoride (PVDF) (10 wt %) using an N-methyl 2-pyrrolidone (NMP) solvent. Then, the mixture was cast onto the aluminium foil (current collector) using a doctor blade. After drying the slurry in a vacuum oven at 120 °C overnight, it was hot-pressed using a twin roller hot press for better adhesion between the slurry and the current collector. The electrochemical cells were assembled using lithium metal foil (reference electrode), glass fiber (separator) and 1M LiPF₆ with a volumetric ratio of EC:DMC of 1:1 (electrolyte) in an argon-filled glove box. The electrochemical properties of the assembled half-cell were investigated on a Wonatech WBCS 3000 electrochemical workstation within the potential window of 1.5–4.5 V vs. Li⁺/Li at different current densities.

3. Results

3.1. Physiochemical Analysis

As can be seen from Figure 1, a total of 58% of weight loss was observed in the TG curve within the temperature range of 200–400 °C, which can be ascribed to the removal of the functional groups of co-ordinated molecules and the carbon skeleton presented in the precursors. Since there was no significant weight loss observed after 400 °C until 700 °C, the as-prepared sample was calcined at 450 °C.

To further confirm the formation of V₂O₅ after calcination at 450 °C, powder X-ray diffraction was performed. The XRD pattern, along with the crystal structure of V₂O₅, is demonstrated in Figure 2a. All of the diffraction peaks were exactly matched with the standard JCPDS data (card no. 00-041-1426, orthorhombic crystal system and Pmmn space group). It can be noticed that the obtained diffraction peaks are highly intense and sharp, which confirmed the crystallinity of the sample without any extra impurity peak. The layered crystal structure of V₂O₅ is also depicted in the inset of Figure 2a.

To further investigate the morphological details of the calcined V₂O₅ sample, FE-TEM was employed, and the obtained results are displayed in Figure 3. As can be seen from Figure 3a,b, the mixed size and shape of the nanoparticles, including the oval-shaped nanoparticles with an average size of ~50–250 nm, are observed throughout the sample. More importantly, there is also less agglomeration, which may be due to the use of the MOF. The typical HR-TEM image in Figure 3b clearly shows lattice fringes with d-spacings of 0.57 nm and 0.34 nm, which correspond to the (200) and (110) planes of the sample. The obtained planes are perfectly matched with the XRD peaks, indicating the high purity of the sample. To further investigate the elemental distribution in the calcined V₂O₅ sample, energy dispersive spectroscopy (EDS) analysis was also performed, and the result is displayed in Figure 2b. The obtained EDS spectra revealed the presence of vanadium (V) and oxygen (O) elements in the sample without any other impurity, which further indicates the fabrication of a pure V₂O₅ sample.

3.2. Electrochemical Performances

To further investigate the electrochemical performances of V₂O₅ as a cathode material, the galvanostatic charge/discharge measurement was first examined at a current density of 250 mA g⁻¹, and the obtained result is displayed in Figure 4a. During the first discharge cycle, numerous irreversible reduction plateaus can be seen, which correspond to the formation of a solid electrolyte interface (SEI) layer as well as multistep electrochemical reactions between Li⁺ and V₂O₅ [35]. From the second cycle onwards, the shape of curves became similar without multistep reduction plateaus being observed in any discharge cycle, indicating the reversible electrochemical reactions within the electrode. The observed significant change in the charge–discharge profiles can be attributed merely to their different electrochemical responses against applied voltages. The V₂O₅ cathode delivered initial discharge/charge capacities of 422.9 and 356.4 mAh g⁻¹, respectively, with a corresponding initial coulombic efficiency of 84.2%. However, as can be noticed, the electrode can deliver a discharge/charge capacity of 267.5/263.9 mAh g⁻¹ after 10 cycles, with a coulombic efficiency of ~99%.

The cyclic voltammetry (CV) profiles of the V₂O₅ cathode at a scan rate of 0.1 mV s⁻¹ are depicted in Figure 4b within the voltage range of 1.5–4.0 V. The first cathodic scan exhibits the multiple irreversible reduction peaks at ~3.35 V, ~3.16 V, ~2.29 V, ~2.23 V, ~2.0 V and ~1.83 V, as observed in the first discharge cycle (Figure 3a), which can be attributed to the multistep lithiation of the V₂O₅ electrode and the formation of the solid electrolyte interface layer. It is well known that the phase transformation occurred during lithiation from α-V₂O₅ to ε-Li_0.5V₂O₅, δ-LiV₂O₅ and then to γ-Li₂V₂O₅, respectively [35,36,37]. However, in the subsequent discharge scans, these multiple reduction peaks completely vanished, and the two main broad peaks located at ~2.4 V and ~3.5 V are observed, which can be attributed to the pseudocapacitive behavior of the V₂O₅ cathode [20]. On the other hand, the two major peaks located at ~2.7 V and ~3.7 V can be seen during the anodic scan, which can be attributed to the reversible phase transformation of V₂O₅. More importantly, from the second cycle onwards, the two pairs of redox peaks between the voltage window of ~2.0–4.0 V can be observed, which correspond to the reversible Li⁺ insertion/de-insertion within the electrode [36]. Generally, V₂O₅ accepts two Li⁺ ions during the discharge and Li₂V₂O₅ forms, which converts back to layered V₂O₅ after the removal of Li⁺ ions during charging. The corresponding reactions can be as follows: [37]

$$\mathsf{\alpha }-{\mathrm{V}}_2{\mathrm{O}}_5+0.5{ \mathrm{Li}}^{+}+0.5{ \mathrm{e}}^{-}\to \mathsf{\epsilon }-{\mathrm{Li}}_{0.5}{\mathrm{V}}_2{\mathrm{O}}_5$$

(1)

$$\mathsf{\epsilon }-{\mathrm{Li}}_{0.5}{\mathrm{V}}_2{\mathrm{O}}_5+0.5{ \mathrm{Li}}^{+}+0.5{ \mathrm{e}}^{-}\to \mathsf{\delta }-{\mathrm{LiV}}_2{\mathrm{O}}_5$$

(2)

$$\mathsf{\delta }-{\mathrm{LiV}}_2{\mathrm{O}}_5+{\mathrm{Li}}^{+}+{\mathrm{e}}^{-}\to \mathsf{\gamma }-{\mathrm{Li}}_2{\mathrm{V}}_2{\mathrm{O}}_5$$

(3)

To further examine the structural stability, the cycling performances were also tested at two different current densities and 250 mA g⁻¹, as shown in Figure 5a. It can be seen that the electrode demonstrates stable cycling performances with the discharge capacities of 169.3 mAh g⁻¹ at 133 mA g⁻¹ and 150.7 mAh g⁻¹ at 250 mA g⁻¹ after 50 cycles. Thus, the oval-shaped V₂O₅ nanoparticles electrode showed excellent capacity retention and a stable cycle life during the charge/discharge reactions, which is possibly due to the unique nanostructure, imparting a high surface area and numerous path channels for the insertion/de-insertion of Li⁺ ions. Moreover, the rate capability of the V₂O₅ cathode material was also investigated at numerous current densities, and the results are displayed in Figure 5b. It can be noticed that the electrode is cycled for 10 cycles at each current density. The electrode delivered discharge capacities of 445.1, 268.15, 198.6 and 128.1 mAh g⁻¹ at the current densities of 133, 266, 532 and 1330 mA g⁻¹, respectively. In addition, when the electrode was reversed back to the lower current density of 133 mA g⁻¹ after 50 cycles of charge/discharge between the lower and higher current densities, it regained a capacity that was similar to the previous capacity, which confirmed the structural stability of the oval-shaped V₂O₅ nanoparticles cathode.

Since various research articles have already been reported in the literature, we have prepared an electrochemical performance comparison table between the current oval-shaped V₂O₅ nanoparticle electrode and the previously reported pure and composite V₂O₅ electrode materials based on numerous nanostructures. It is concluded from

Table 1. Electrochemical performance comparison between the current oval-shaped V₂O₅ nanoparticle electrode and the previously reported pure and composite V₂O₅ electrode materials based on numerous nanostructures.

Material	Synthesis Method	Nanostructure	Electrochemical Performance (Dis. Capacity/Cycle Number/Rate)	Ref.
V2O5	The polymer-assisted chemical solution method	Network	99 mAh g−1/100 cycles/1 A g−1	[38]
V2O5	Solvothermal	Sheets	140 mAh g−1/100 cycles/0.1C	[39]
V2O5/MWCNT		Particles	190 mAh g−1/100 cycles/0.1C
V2O5	Hydrothermal	Nanobelts	168 mAh g−1/50th cycles/50 mA g−1	[40]
V2O5	The ultrasonic method with subsequent thermal decomposition.	Nanosheets	179.5 mAh g−1/50 cycles/1C	[37]
		Nanoparticles	106.1 mAh g−1/50 cycles/1C
V2O5	Hydrothermal/annealing	SLMNSs	202 mAh g−1/50 cycles/100 mA g−1	[41]
		Nanoribbons	131 mAh g−1/50 cycles/100 mA g−1
V2O5	Microwave-assisted hydrothermal synthesis	Separate nanorods	112.6 mAh g−1/50 Cycles/100 mA g−1	[42]
		Nanorods assemblies	191.6 mAh g−1/50 cycles/100 mA g−1
V2O5-SnO2	Solvothermal	Double-shelled nanocapsules	174 mAh g−1/50 Cycles/100 mA g−1	[20]
V2O5	Hydrothermal	Nanowire	126 mAh g−1/100 Cycles/100 mA g−1	[43]
V2O5/rGO		Quantum dots	212 mAh g−1/100 Cycles/100 mA g−1
V2O5	Solvothermal	Hollow microspheres	125 mAh g−1/100 Cycles/1 C	[44]
V2O5	Solvothermal	Nanoparticles	91.40 mAh g−1/100 Cycles/50 mA g−1	[45]
V2O5	Hydrothermal	Nanoflakes/ Nanoparticles	139 mAh g−1/50 Cycles/15 mA g−1	[46]
V2O5	Hydrothermal	Oval-shaped Nanoparticle	169.3 mAh g−1/50 cycles/133 mA g−1	This work
			150.7 mAh g−1/50 cycles/250 mA g−1

that the oval-shaped V₂O₅ nanoparticle electrode provides a comparable electrochemical performance and can be considered a potential cathode material for LIB.

4. Conclusions

This work demonstrates a metal-organic framework that fabricated an oval-shaped V₂O₅ cathode material by using a simple and facile hydrothermal synthesis method. It is believed that the obtained unique nanostructure of the V₂O₅ electrode may provide a high surface area and numerous channels for Li⁺ insertion/de-insertion, resulting in improved electrochemical performances. When applied as a cathode material, the electrode exhibits discharge capacities of 169.3 mAh g⁻¹ at 133 mA g⁻¹ and 150.7 mAh g⁻¹ at 250 mA g⁻¹ after 50 cycles and 57.5 mAh g⁻¹ at a high current density of 1330 mA g⁻¹ after 40 cycles. The obtained electrochemical performances showed that the oval-shaped V₂O₅ electrode is a potential cathode material for LIBs, and the current synthetic strategy can be also extended to the fabrication of other electrode materials.